1. Field of the Invention
The present invention relates to an injection molding machine that controls each operational process in a molding cycle by variably controlling the number of revolutions of a drive motor in a hydraulic pump, and a control method thereof.
2. Description of the Relevant Art
Conventionally, an injection molding machine equipped with a hydraulic drive member where the number of revolutions of a drive motor in a hydraulic pump is variably controlled, and that drives & controls hydraulic actuators, such as an injecting cylinder (hydraulic cylinder), based upon the number of revolutions. At the same time, the control of each operational process in a molding cycle is known in Japanese Patent Publication No. 3245707.
The injection molding machine (and the control method thereof) disclosed in the publication target improved stability of the pressure control by eliminating the fluctuations of control pressure at the time of pressure control, and it is designed such that the discharge flow rate and discharge pressure of the hydraulic pump are controlled by controlling the number of revolutions of a servomotor in a fixed discharge type hydraulic pump. At the same time, hydraulic oil discharged from the hydraulic pump is relieved to an oil tank by a relief circuit especially in order for the number of revolutions of the hydraulic pump in the fixed discharge type hydraulic pump to be always the number of revolutions away from an unstable region of the rotational resistance in the hydraulic pump, or greater.
However, the control method for the conventional injection molding machine (hydraulic drive unit) had the following problems to be resolved:
First, a large-sized servomotor is required in response to the capacity of the injection molding machine (maximum capacity) in order to control the discharge flow rate and the discharge pressure by controlling the number of revolution of the servomotor. Therefore, as a measure against an unstable region where the number of revolutions of the servomotor becomes smaller, when the hydraulic oil to be discharged from the hydraulic pump is relieved to an oil tank, the energy waste becomes great and it is disadvantageous from the standpoint of energy savings and running costs.
Secondly, since a large-sized servomotor is required in response to the maximum capacity of the injection molding machine, the servomotor itself becomes expensive, increasing the initial cost. In particular, the upsizing of the servomotor creates a greater capacity (greater electric current) of an attached servo circuit (servo amplifier), and ensures that the ability to withstand electric power in the servo circuit gradually increases the overall cost.
Thirdly, since all operations in the injection molding machine are controlled by a single servomotor, a region where the operational capability of the servomotor does not conform with each operational process of the injection molding machine easily occurs. Therefore, control tends to be unstable, disadvantageous from the standpoint of securing moldability and molding quality. At the same time increased overload frequency is disadvantageous from the standpoint of reliability and longer life, as well.